Re: [PATCH 02/11] x86/vmx: Don't clobber %dr6 while debugging state is lazy

["Jan Beulich" <JBeulich@suse.com>]

>>> On 08.06.18 at 17:58, <andrew.cooper3@citrix.com> wrote:
> On 07/06/18 12:05, Jan Beulich wrote:
>>>>> On 06.06.18 at 16:16, <andrew.cooper3@citrix.com> wrote:
>>> On 06/06/18 14:50, Jan Beulich wrote:
>>>>>>> On 04.06.18 at 15:59, <andrew.cooper3@citrix.com> wrote:
>>>>> --- a/xen/arch/x86/hvm/vmx/vmx.c
>>>>> +++ b/xen/arch/x86/hvm/vmx/vmx.c
>>>>> @@ -3696,6 +3696,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
>>>>>               */
>>>>>              __vmread(EXIT_QUALIFICATION, &exit_qualification);
>>>>>              HVMTRACE_1D(TRAP_DEBUG, exit_qualification);
>>>>> +            __restore_debug_registers(v);
>>>>>              write_debugreg(6, exit_qualification | DR_STATUS_RESERVED_ONE);
>>>> The change is certainly correct as is, but I'd still like to put out for
>>>> discussion the alternative option:
>>>>
>>>>     if ( v->arch.hvm_vcpu.flag_dr_dirty )
>>>>         write_debugreg(6, exit_qualification | DR_STATUS_RESERVED_ONE);
>>>>     else
>>>>         v->arch.debugreg[6] = exit_qualification | DR_STATUS_RESERVED_ONE;
>>>>
>>>> After all the guest may know it's single stepping, and may not care to
>>>> read DR6 at all.
>>> All of this code changes across the series (so this specific suggestion
>>> is incorrect), but to the recommendation in general...
>> While I've not made it through the second half of the series yet,
>> another consideration: To avoid the double DR6 write, yet still avoid
>> an immediate further exit to restore debug registers, why not
>>
>>     if ( v->arch.hvm_vcpu.flag_dr_dirty )
>>         write_debugreg(6, exit_qualification | DR_STATUS_RESERVED_ONE);
>>     else
>>     {
>>         v->arch.debugreg[6] = exit_qualification | DR_STATUS_RESERVED_ONE;
>>         __restore_debug_registers(v);
>>     }
> 
> Do you really think the added complexity is worth shaving a few cycles
> off an almost unused cold path?  The double %dr6 write (which isn't
> serialising) happens at most once per vcpu context switch, and only ever
> when the guest is debugging.

Yeah, you're right - keep it as is.

Jan



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